1. Field
The following description relates to a soft demapping apparatus for a system using a signal space diversity (SSD) scheme.
2. Description of Related Art
Research is being conducted on signal space diversity (SSD) for improving a bit error rate (BER) in a fading channel.
To obtain the SDD, a quadrature amplitude modulation (QAM) signal may be transmitted by rotating the QAM signal and by interleaving an in-phase (I) channel and a quadrature (Q) channel with respect to each other. In this example, a transmission signal of the I channel and a transmission signal of the Q channel may have different channel gains such that a receiver may obtain a diversity.
In particular, the receiver may obtain a significant performance gain in an erasure channel in which one of the I channel and the Q channel is erased, or in a zero decibel (dB) echo channel.
As a simple interleaving method, a rotated or tilted QAM signal having a Q channel delay may be used. This method has been adopted and employed as a standard in a digital broadcasting system and the like.
However, in such system, due to the rotation, a correlation may occur between the I channel and the Q channel. Accordingly, an amount of calculations of a soft demapper, for example, an amount of log likelihood ratio (LLR) calculations may be increased.
When the receiver uses a maximum likelihood (ML) scheme, two pulse amplitude modulations (PAMs) may be regarded as being received because the I channel and the Q channel are independent from each other in a non-rotated QAM. For example, in the case of 2m QAMs, 2×22/m one-dimensional (1D) LLR calculations may be used.
Also in this example, a distance corresponding to 2m may be used. Accordingly, in the case of a high modulation, a receiver may use most of the calculation amounts for a soft demapping algorithm. Because of the different channel gains between the I channel and the Q channel, a two-dimensional (2D) Euclidian distance calculation and the number of Euclidian distance calculations corresponding to the number of signals may be used to calculate an LLR.
Accordingly, a soft demapping algorithm using fewer calculations may be desired to decrease the complexity of the receiver. Even though max-log approximation is proposed to decrease the complexity of the receiver, the amount of 2D distance calculations may correspond to the number of full search rotated signals.
Methods for dividing a region into sub-regions have been proposed to decrease an amount of calculations, and thus, an amount of calculations with respect to the full search has significantly decreased. However, in the case of high modulation, a significantly large amount of calculations are still required and degradation in performance to the full search may occur.